Electron tube protective system



Oct. 9, 1951 L. P. GARNER 2,571,027

ELECTRON TUBE PROTECTIVE SYSTEM Filed March 9, 1950 lllllll INVENTOR LloYd P Garner ATTORNEY PATENT OFFICE ELECTRONTUlSI llROTECTIVE SYSTEM I jiloyd PnGarnerr ...-,w re. .11 r

'W'Ihis invention relates to improvements. in .pro.+ tectlve systems for-electron tubes,.and.particularly to a system for protecting such: tubes against faultscausing abnormal tube current flow.

Presently known high power electron tubes are.

designed to handle as much as amillion .watts of:

currents for thetime required to operate con--v ventional overload protection devices"; .However,

a serious problem arises in the operation of such tubes when a tube faultcauses abnormallyhigh current to flow to a localized spot on one or more of. the tube electrodes. Inthe. usual case, such faults take the form of an arc discharge through stray-quantities of gas in the tube. Occasionally aninternal short may develop between two tube electrodes, such as when one of the electrodes 7 becomes sufficiently deformed due to localized overheating or the like to make contact with another tube electrode. In the case ofa gas arc, the tube need not be damaged at all if the arc canbe stopped quickly enough, while in the case ofan internal short, it is possible tosave mostof the tubeelements if the source of power can be. removed quickly enough. In either, event, how'- ever, the high cost of such tubes makes it impera-v tive to provide the maximum possible protection against these potentially destructive faults.

Conventional prior art protective systems gen erally consist of some arrangement for interrupting or shutting ofi the power supply to the protected tube upon the occurrence of a faulttherein. Such systems have been found incapable of adequately protecting tubes against localized high currents since they cannot respond rapidly enough to remove the energizing voltage before the tube has been ruined. For example, the fastest-acting electromechanical circuit breakers require several hundredths of a second to operate after an initiating impulse is received, and arcs at the breaker terminal may increase the actual circuit-opening time. More rapid power-cutoff action can be obtained with a power supply'of the grid controlled rectifier type, (see e. g. Mercury Arc Power Rectifiers, Monti and Winograd, Mc- Graw-Hill Book Company, 1930, pgs. 397-402),- wherein the rectifier output can be cut off by applying a negative voltage to the rectifier grids. Even in this case, the cutoff time is not less than that required for the completion of a given half cycle of rectifying action. With a 60 cycle alternating cur-rentsupply, forexample, then'cutoff,

ancaster, Pa.,.assignor to Radio Coyporationof America, a corporation of Dela- 1950;"Serial No. 148,723

Cl. 315T349 time is of the order of one hundredth of a second;- Since localized high current in a tube handling," many kilowatts of power can totally destroy the tube in a matter of microseconds, it can beseen I that conventional protective systems of the'circult-opening typeare incapable of providingthe' necessary protection.

It is, accordingly, a general object of the present invention to provide an improved systemfdrprotecting high power electron tubes against high-current faults. Y 7

A more specific object of the invention is to provide a tube protection system which will actsubstantially instantaneously to arrest the flow of power to a tube upon the occurrence of a fault therein.

' In accordance with the invention, the foregoing and other related objects and advantages are attained by providing a normally open substantiallyshort-circuit path in parallel with the power supply through which current can flow upon the occurrence of a fault in a protected tube. Stated somewhat differently, the invention provides means for substantially instantaneously diverting high current from an inherently delicate electron tube by a circuit closing action upon the occurrence of a fault therein, as distinguished from relatively slow-acting circuit opening apparatus Fault detecting means are coupled to the circuit of'- the protected tube for initiating the circuit closing action. 1

A preferred short-circuiting element for use in the protective system of the present invention: comprises a gas tube having two principal elec trodes (e. g. anode and cathode) and an auxiliary firing-control element (e. g. a control grid or an igniter electrode). As used herein and in the appended claims, the term gas tube is intended to 'mean an electron tube adapted to withstand relatively high voltages when nonconductive, and containing gas or vapor in such quantity that,

when the tube is conducting, the voltage drop thereacross will have a low, substantially constant value characteristics of a gas or vapor discharge ('e. g. a thyratron or an ignitron). This'is in comparison with evacuated electron tubes in which the conducting voltage drop varies ap-. proximately as the two-thirds power of the tube current. The term firing is used herein and in the appended claims to denote initiation of cur rent flow in a gas tube. A more complete understanding of the inven-.- tion can be had by reference to the accompanye- 5 5- ing drawing,- the single figure otwhichillustrates anode I8 through a reactor 24, and the tube a cathode 26 being connected to the negative terminal 28 of the power supply 22 through a resistor 30. The purpose of the reactor 24 and the resistor will be described hereinafter; A bias voltage source, shown as a battery 32, is connected between the tube grid l2 and the cathode 26, in series with a grid resistor 34.

A resonant circuit 36 comprising a parallel connected inductor 38 and capacitor 40 is connected to the tube anode 13 through a blocking capacitor 42 in the usual manner, so that amplified alternating voltages will be made available at an output terminal Mi. Although a conventional amplifier circuit has been shown for simplicity of illustration, it will be understood that the tube It could as well be connected in an oscillatory circuit by providing suitable feedback connections between the tube anode i8 and the tube grid I2. In either case, the tube in will act as an alternating voltage amplifier. Similarly, the protection of a tube in a high power modulation circuit or in a so-called buffer stage is deemed to be within the scope of the present invention.

The power supply 22 is provided with a pair of input terminals 46 through which a suitable alternating voltage can be applied to the rectifier. It is assumed that the power supply 22 is capable of furnishing a unidirectional voltage of the order trolled rectifiers of the type previously referredto. As was mentioned, such overload protective devices cannot respond rapidly enough to protect the tube I 5 upon the occurrence of a localized high-current fault therein.

In accordance with the invention, fault protection for the tube It is provided by a normally non-conductive short-circuiting element consisting of a gas tube connected in parallel with the power supply 22. As shown, the gas tube anode 52 is connected directly to the positive power supply terminal 26, and the gas tube cathode 54 is connected directly to the negative'terminal 28 of the power supply. The tube 55 is a so-called grid-controlled thyratron, in which firing iscontrolled by a grid electrode 55.

The fault detecting means provided in the embodiment shown comprises the resistor as connected in the protected tube circuit between the tube cathode 26 and the power supply terminal 28. The resistor 39 also is connected in the gas tube grid circuit in series with a bias voltage source, shown as a battery 58. The voltage of the bias source 58 is made suiilciently negative to prevent firing of the tube 5!] during normal current flow through the protected tube In. However, upon the occurrence of a high-current fault, such as an are or short-circuit, in the tube 10, the voltage developed across the resistor 30 will be of sufiicient magnitude to overcome the bias voltage of the source 58, driving the gas tube control grid 56 positive with respect to the cathode 54 to fire the tube 55. This action, which will occur within a very few microseconds after the beginning of a fault in the tube It], will divert the current away from the tube It) and through the tube 50 provided the conducting impedance of the tube 55 is less than that of the tube I0 after a fault develops. Under these circumstances, the voltage between the anode l8 and the cathode 25 of the tube It will be insufficient to sustain damaging current flow, and the tube l0 will be completely protected.

In order to insure that there will be sufiicient voltage across the tube 50 to cause firing thereof upon occurrence of a fault in the tube It, an impedance such as the reactor 24 preferably is connected between the tube anode l8 and the gas tube anode 52. The reactor 2 will serve to hold up the voltage on the gas tube anode 52, when a fault develops in the protected tube I0, long enough for the gas tube 5%? to fire. It will be understood that the reactor 25 can as well comprise a resistor, although the use of a resistor will involve some power loss.

From the point of view of the power supply 22, a short-circuit in the form of current flow through the gas tube 55 is as undesirable as an are or short-circuit in the protected tube l0. However, the protective devices in the power supply normally will operate rapidly enough to prevent damage since there ordinarily willbe no localization of heavy current in the power supply as in the case of a fault in the tube It.

It will, of course, be apparent that the protective system described herein will respond to and provide protection against abnormal current flow in the protected tube whether or not the current flow islocalized with respect to one or more of the tube electrodes. While the present system is primarily important for the protection it will provide against localized abnormal currents which will destroy a tube if not eliminated almost instantly, it also will provide effective protection against unlocalized abnormal currents such as would accompany loss of grid bias or the like. Thus, while such unlocalized currents might be cut off by a circuit breaker before totally destroying the tube, more prompt protection will reduce the cumulative effects of repeated occurrences of unlocalized high currents and help to increase tube life.

As was previously mentioned, the present invention is not limited to the use of grid controlled gas tubes, as there are other well known firing control arrangements any one of which can be adapted for use in the present invention (see e. g. Reich-Theory and Application of Electron Tubes, McGraw-Hill Book Company, 1930, pgs. 393-398). Special mention may be made of the mercury-pool cathode ignitron tube, in which firin is controlled by an igniter electrode partially immersed in the cathode pool. The ignitron tube is preferable for very high power applications because of its large current carrying capacity. Examples of the use of ignitron tube in protective systems embodying the principles of the present invention will be found in the co: pending application of W. N. Parker et -al., Serial No. 148,688, filed March 9, 1950, and assigned to the same assignee as the present invention. Modified arrangements for detecting tube faults and for firing the protective gas tube also are described and claimed in the foregoing Parker et al. application.

Since these and other changes could be made in the specific embodiment of the invention described herein, the foregoing is to be construed as illustrative, and not in a limiting sense.

What is claimed is:

1. In a protective system for an electron tube energized from a high voltage source, in combination, a normally nonconductive short-circuiting element comprising a gas tube connected in parallel with said voltage source, current responsive voltage generating means in circuit with said electron tube for generating a firing voltage of predetermined magnitude and polarity in response to the flow of current greater than a predetermined value through said electron tube, and means coupling said voltage generating means to said gas tube to fire said gas tube in response to said firin voltage.

2. A protective system for an electron tube connected to amplify alternating voltages and energized from a unidirectional voltage source, said system comprising a circuit connected in parallel with said voltage source and consisting of a normally non-conductive gas tube having an anode, a cathode, and a firing control electrode, current responsive voltage generating means in circuit with said electron tube for generating a voltage impulse of magnitude sufiicient to fire said gas tube in response to abnormal current flow through said electron tube, and means coupling said voltage generating means to said firing control electrode to fire said gas tube upon the occurrence of a fault in said protected tube.

3. In an electron tube protective system, the combination with an electron tube connected to amplify alternating voltages and energized from a unidirectional voltage source, of a gas tube connected in parallel with said source and having an electrode for controllin firing of said tube, a resistor connected in series between said electron tube and said source, and a circuit connecting said resistor to said electrode to fire said gas tube in response to abnormal current flow through said electron tube and said resistor.

4. In a protective system for an electron tube connected to amplify alternating voltages and supplied with unidirectional operating voltage from a high voltage source, in combination, a

gas tube having an anode, a cathode, and a control grid, a circuit connecting said tube in parallel with said voltage source, a resistor connected between said electron tube and said voltage source, and a circuit connecting said control grid to said cathode and including said resistor and a voltage source for maintaining said control grid suiilciently negative with respect to said cathode normally to prevent current flow in said gas tube.

5. A protective system for an electron tube energized from a high voltage source, said system comprising a circuit connected in parallel with said voltage source and including a normally non-conductive element capable of withstanding relatively high voltage when non-conductive and having a low substantially constant voltage drop thereacross when conducting current, means to render said element conductive in response to a voltage impulse of predetermined magnitude, a resistor connected between said electron tube and said voltage source, and a circuit connecting said resistor to said means to furnish to said means voltage developed across said resistor by tube current, said last named circuit including a voltage source of polarity opposite to that of said voltage across said resistor.

6. A protective system for an electron tube energized from a high voltage source, said system comprising a circuit connected in parallel with said voltage source and including a normally non-conductive element capable of withstanding relatively high voltage when non-conductive and having a low, substantially constant voltage drop thereacross when conducting current, means to render said element conductive in responsive to a voltage of predetermined magnitude, current responsive voltage generating means in circuit with said electron tube for generating a voltage of said predetermined magnitude in response to the flow of current of greater than predetermined magnitude through said tube, and means coupling said voltage generating means to said first-named means to render said element conductive in response to said greater than predetermined magnitude current flow through said tube.

LLOYD P. GARNER.

(No references cited.) 

